comma position in a list of subjects

Question

In the following sentence, I mean policy makers and decision makers. It is not clear to me how to express this and where to place the comma:

If I use this:

This study benefit vendors, policy and decision makers

Or

This study benefit vendors, policy, and decision makers

Answer 1

If the policy and decision makers are a single entity then I'd write it like this...

This study benefits vendors, and policy and decision makers

If they are two separate entities then its clearer to write it like this using an Oxford comma...

This study benefits vendors, policy makers, and decision makers

The Oxford comma can be used as the final comma in a list of 3 or more things so that the reader doesn't confuse the last two items in the list as being combined.

This is a good example I found on Grammarly.com:

"I love my parents, Lady Gaga and Humpty Dumpty"

Without the Oxford comma, the sentence above could be interpreted as stating that you love your parents, and your parents are Lady Gaga and Humpty Dumpty. Here’s the same sentence with the Oxford comma:

"I love my parents, Lady Gaga, and Humpty Dumpty"

https://www.grammarly.com/blog/what-is-the-oxford-comma-and-why-do-people-care-so-much-about-it/

Answer 2

You should use the second form in a modified version

This study benefits vendors, policy makers, and decision makers

You should also use benefits over benefit. If you want to use "benefit" it would make more sense to say "This study is of benefit to vendors,policy makers,and decision makers".